Brown off the lamb shanks in a pan and transfer to an ovenproof dish along with the carrot, celery, onion, garlic and rosemary. Pour in the water and season to taste.
Cover dish and put into a preheated 180 C / Gas 4 oven for 3 to 4 hours.
Once the meat starts to fall from the bone, take the shanks out of the dish and rest them.
Pour the liquid and vegetables into a saucepan, remove the rosemary and blitz with a stick blender until smooth. Bring the gravy to a simmer and add flour until you reach your desired thickness. Season with salt and pepper to taste.
Serve the shanks with your favourite vegetables along with your gravy. Enjoy!